1. Field of the Invention
This invention relates generally to a system for producing melt-phase polyesters. In another aspect, the invention concerns an esterification system for use in a polyester facility capable of producing a low-impurity polyester product.
2. Description of the Prior Art
Melt-phase polymerization can be used to produce a variety of polyesters, such as, for example, polyethylene terephthalate (PET). PET is widely used in beverage, food, and other containers, as well as in synthetic fibers and resins. Advances in process technology coupled with increased demand have lead to an increasingly competitive market for the production and sale of PET. Therefore, a low-cost, high-efficiency process for producing PET is desirable.
Generally, melt-phase polyester production facilities, including those used to make PET, employ an esterification stage and a polycondensation stage. In the esterification stage, polyester raw materials (i.e., reactants) are converted to polyester monomers and/or oligomers. In the polycondensation stage, polyester monomers and/or oligomers exiting the esterification stage are converted into a polyester product having the desired final chain length.
In most conventional melt-phase polyester production facilities, esterification is carried out in one or more mechanically agitated reactors, such as, for example, continuous stirred tank reactors (CSTRs). However, CSTRs and other mechanically agitated reactors have a number of drawbacks that can result in increased capital, operating, and/or maintenance costs for the overall polyester production facility. For example, the mechanical agitators and various control equipment typically associated with CSTRs are complex, expensive, and can require extensive maintenance. Further, conventional CSTRs frequently employ internal heat exchange tubes that occupy a portion of the reactor's internal volume. In order to compensate for the loss in effective reactor volume, CSTRs with internal heat exchange tubes require a larger overall volume, which increases capital costs. Further, internal heat exchange coils typically associated with CSTRs can undesirably interfere with the flow patterns of the reaction medium within the vessel, thereby resulting in increased impurity levels and an overall loss of conversion. To increase product conversion, many conventional polyester production facilities have employed multiple CSTRs operating in series, which further increases both capital and operating costs.